Composite piston



H. H. DILLON- COMPOSITE PISTON March 30, 1943.

Filed Sept. 24, 1940 INVENTOR.

u N. W

H 7 m%fi. H

Patented Mar. 30 943 2,315,403 COMPOSITE PISTON- Harold H. Dillon,Cleveland, O'hio,assignor, by

mesne assignments, to Aluminum Gompanyci America,

Pennsylvania Pittsburgh, Pa.,'-a' corporation of Application September24, 1940, Serial No. 358,117

aoiajims.v (chaos-14) This invention relates broadly to pistons forinternal combustion engines and more specifically to pistons comprisingseparately skirt portions.

The most commonly used pistons for internal formed head and g tcombustion engines have been integral one piece castings ofaluminumalloy in orderto obtain, at a practical cost, the strength," ruggednessand durability essential for operation, combined with the advantages ofthe light weight and high'heat conductivity of aluminum alloy. Theproblems that have arisen in connection with such pistons I flow mainlyfrom'the relatively high coeflicient of thermal expansion of thealuminum alloy as compared with the cast iro'n'f'rom which the env ginecylinders are commonly made.

The principal object of the present invention is to combine in apractical manner a piston head portion, which may be a casting ofaluminum or other light metal alloy, and a skirt portion which may beformed from sheet'steel or other ferrous metal in such a manner thattheassembled pis'- tori will be able'at'all times to properly transmitandwithstand the explosion, inertia, and thrust forces to which it issubjected; a close clearance will be maintained at all temperaturesbetween the skirt or guiding portion of the piston and skirt at anyoperating temperature; and'by providingthe upper end of the skirt andthe head with angularly disposed abutting surfaces arranged so as toremain in firm sliding contact without stressing 'or distorting theskirt at all operating temperatures and acting to prevent relativesliding movement of the head and skirt portions along the wrist pin axisor relative tilting of the parts about this axis. In the preferredembodiment of the invention the abutting surfaces on the skirt and headare conical and arranged at an angle proportional to the ratio betweenthe radius of a point on the conical surfaces andgthe vertical distanceofthe same from V a horizontal plane through the wrist pin axis.

Referring to the drawing wherein'the preferred v embodiment of theinvention is'illus'trated:

Fig. 1 is an elevational view of the piston embodied in the presentinvention with parts thereof shown in section for the sake of clarity;

Fig. 2'is a vertical sectional view ofthe piston showing the angle ofthe inclined surface between the cylinder wall; and distortion orstressing of I the skirt from the different thermal expansion andcontraction of the material of the head portion will be prevented. Otherobjects are to eonnect the two portionsof the pistontogether m a simple,practical and economical .manner; to

- utilize the wrist pin by which the piston is connected to the usualconnecting rod for holding the two parts of the piston together withoutsubjecting the skirt to distorting forces resulting:

irom'the relative movement, during expansion and contraction, of thewrist pin bosses carried by the-head; and to center the skirt portionwith respect to the head portion and hold the two parts against relativemovement along the wrist pin axis and against relative tilting about thewrist pin axis by engagement-of the upper end of the skirt with thehead, without imposing stresses on the skirt portion or permittingclearance to develop betwen the skirtancl head during the difierentialexpansion and contraction of the parts. v

, These and other objects whichwill appear hereinafter are attained byproviding the head bosses receiving a wrist pin, with the skirt bossesspaced sumciently from the head bosses to prevent the head bosses fromexerting pressure on the and. skirt portions. of the piston with alignedthe head and skirt portion with a plane through the'center of .the wristpin bosses, the section be ng taken on a line 2-2 of Fig. 1;

Fig. 3 is a transverse sectional view of the piston showing the wristpin bosses in: the skirt.

spaced from the bosses formed in the depending webs or pillars of thehead portion, the section "being taken on a line 3 3 of Fig. 1; and,

Fig. 4 is a diagrammatic view: of the piston showing the method ofdetermining the angle of the inclined surfaces.

Referring to Fig. 2. the piston disclosed therein comprises a separatelyformed upperorheadpor- The head tionlll and a lower or skirt portion ll. portion has cast integral'therewith webs l5 and depending pillars l2which have wrist p'in bosses l3 formed in the loweriends thereof.

In the formation of the piston it is desirable that the headand integralpillars H be formed of a light, strong and durable metal which possessesa relatively high heat conductivity. Thus it is preferable that the headand pillar portions .of

the piston be cast ofsome high expanding metal such'as aluminum or analuminum alloy. A plural iy f grooves I! are provided in the peripheralsurface of the piston-headfor the reception of V the conventional pistonrings, not shown. Formed on the lower circumferential edge of the headsubjacent-the lowermost ring grove I there is an inclined or taperedsurface l6 which is adapted to engagea complementalsurface pro,- videdon the upper edge of the skirt H,

The skirt portion l! which is formed by press-1 ing, spinning or weldinga low expanding metal such as steel, encompasses the depending pillarsl2- and is disposed in abutting relation with the head ll. Formed inopposed sides of the skirt ortion and in axial alignment with the bossesI! in the pillars I! are wrist pin bosses I! which have steel bushingsI! inserted therein and brazed thereto. Pressed within the steelbushings II are bronze bushings I! which are adapted to receive thewrist pin 22. The wrist pin also extends through the bosses II to holdthe head and skirt together, in addition to providing for a pivotalconnection of the piston to the connecting rod of the engine in theusual manner. The outer edges of the bosses I! are separated from theinner edges of the bosses H by spaces 20. The upper circumferential edgeof the skirt has an inturned. inclined surface 2| formed thereon uponwhich the tapered surface I! of the head is disposed to provide for asliding engagement between the head and skirt portions.

In the manufacture of the piston the head is preferably cast in apermanent mold from aluminum or other light metal alloy and the wristpin bosses II in the pillars I! are cored out in the usual manner. Theinclined surface I on the skirt and the mating surface II on the headare ilnish machined separately at the desired angle before finishing theopenings in the bosses.

II or I! and prior to the finishing of the exterior surface of theskirt. The parts arethen assembled with the mating inclined surfaces I.and 2! pressed together into firm yet slidable contact in substantiallythe relation desired in the completed piston, and while the head andskirt portions are held in their desired concentric relation in asuitable jig or fixture the holes in both sets of wrist pin bosses IIand H are finish bored simultaneously. The piston is then assembled witha snug wrist pin in place after which the exterior surface of the skirtis ground to the desired diameter and finish.

By finish boring the bosses in both portions of the piston at the sametime, in this manner, the desired contact between the inclined surfacesis obtained with theaxes of the two sets of bosses in exact alignment,providing a true surface contact and a free sliding movement between thehead and skirt at all temperatures in the completed piston. The inclinedsurfaces on the head and skirt are pressed together, preferably withjust sufficient force to hold the head against rocking or slidinglaterally with respect to the skirt, which is guided in the cylinder,and with insumcient force to distort the skirt or hinder the sliding ofthe surfaces during expansion and contraction.

The proper inclination of the surfaces I and II with respect to thewrist pin axis is determined by the ratioof the vertical height of thepillars II, from the wrist pin axis to a point on the inclined surfaces,divided by the radius of the piston at that point. During heating, thepillars expand vertically, causing the wrist pin and the skirt to movedownwardly with respect to the head. At the same time the head expandsradially, tending to move the inclined surface it of the head radiallyoutward. The skirt, being composed of a material of a lower coefilcientof expansion, expands both vertically and radially to a lesser extentthan the head. With the inclined surfaces disposed at the proper angle,the two expansions act to slide the inclined surface of the head overthe inclined surface cf the skirt without increasing or decreasing thecontact pressure between these surfaces. Theoretically, assuming auniform heating of all parts, the correct inclination is reached whenthe tangent of the angle of the inclined surfaces from the horizontal isequal to the ratio of the height from a horizontal plane through thewrist pin axis to any point on the inclined surface and the radius ofthe piston to said point on the inclined surface.

I This relationship is illustrated in Fig. 4 of the drawing. In theparticular embodiment illustrated, the angle a of the inclined surfaceswith the wrist pin axis is approximately 40 while the angle 1: of theinclined surfaces with the vertical axis of the piston is approximately50. Thus with the inclined surfaces disposed at such an angle the heightof the pillars, designated as d, from the wrist pin axis to a point 0 onsaid surfaces is the tangent of angle a multiplied by the radius,designated 1, of the piston to the point c.

This relationship remains true and determines the proper angleofinclination for any size or relative proportionof the piston. However,in certain designs the operating temperature of the webs may varyslightly from the operating temperature of the head, requiring acorresponding adjustment of the angle of inclination.

The space 2| between the wrist pin bosses l3 and the bosses II in theskirt permits the bosses I! to be carried outwardly along the wrist pinby the diametrical expansion of the head without imposing any stressupon the skirt. The pillars I! are made relatively rigid with the headin order to properly withstand and transmit the explosion and inertiaforces between the head disc and the wrist pin. Thus the bosses ii areseparated during expansion in accordance with the diametric'al expansionof the head disc, while the skirt bosses i1 move apart in accordance Iwith the lesser'expansion of the skirt. Such relativemovement of thebosses I! along the wrist pin axis is permitted because the bosses IIare freelyrotatable and slidable longitudinally on the wrist pin. Thebosses it are held equally spaced or centered with respect to the skirtbosses IT at all temperatures by the engagement of the inclined surfacesI6 and 2| which, as previously explained, remain in contact at alltemperatures not only to hold the head and skirt against relativetilting movement but also to hold the skirt centered with respect to thehead. The wrist pin may be held from sliding out of the piston intoengagement with the cylinder wall by being secured'to the connecting rodor may be of the full floating type with suitable means (not shown) toprevent the pin escaping from the bosses ll.

By this construction a strongand durable piston is provided in which theinertia and explosion forces transmitted between'the head and the wristpin are carried by the pillars l2 integral with the head portion andbearing directly on the wrist pin through the bosses If. The lateralthrust forces are transmitted from the wrist pin directly to the skirtbearing in the cylinder through the bosses l1 bearing upon the wristpin. The skirt is preferably made of sheet steel or other ferrous metalso that its coeflicient of thermal expansion is substantially the sameas that of the cylinder. Thus the skirt can be ground to a substantiallycircular sectional contour with but a small clearance from the cylinderwall and this close clearance will be maintained skirt portions aresecured together solely by the wrist pin while the sliding joint betweenthe inclined surfaces I6 and 2| as previously explained maintains theparts in correct concentric relation without transmitting any'of thelarge forces to which the piston is subjected.

Although the foregoing description is necessarily ofa detailedcharacter, in order that the o I invention may be completely set forth,it is to be understood that the specific terminology is not intended tobe restrictive or confining, and that variousrearrangements of parts andmodifications of detail may be resorted to without departing from thescope or spirit of the invenand holding said skirt and head together,said head and skirt being formed with inclined complemental engagingsurfaces disposed above said wrist pin bosses, said surfaces beinginclined at an angle the tangent of which is proportional to the ratioof the height of a point on the inclined surface above the wrist pinaxis to the distance from said point to the axis of the piston, y 2. Apiston comprising a head having depending pillars carrying wrist pinbosses, a separate skirt having aligned wrist pin bosses formed andholding said skirt and head together, said head and skirt being formedwith inclined compiemental engaging surfaces disposed above said wristpin bosses, said surfaces being inclined at an angle the tangent ofwhich is theratio 01' the distance of a point on the inclined surface toa horizontal plane through the wrist pin axis, to the radius ofthepiston to that point, the outer edges of the said bosses carried bythe head being spaced inwardly along the wrist pin from the inner edgesof the bosses formed in said skir 10 when the piston is cold. v

3. A piston comprising a head having depend ing pillars carryingwrist-"pin bosses formed of a single piece of aluminum alloy, and aseparate skirt formed of ferrous metal having aligned wrist pin bosses,the upper portion of said skirt having an inclined bearing surfaceengaging a;

complemental bearing surface formed on the head, and a wrist pin passingthrough the'wrist outer edges of the bosses carried by the head be- 1'ing spaced inwardly along'the wrist pin from pin bosses carriedflbysaid head and the bosses formed in said skirt and constituting the solemeans holding the head and skirt together, the

the inner edges of the bosses formed in the skirt when the piston iscold, andsaid inclined bearing surface being arranged at such an anglethat the difierential radial expansion of said head and said skirt iscompensatedby the differential axial therein, a wrist pin passingthrough said bosses expansion of said skirt and said pillars, so thatsaid bearing surfaces maintain said skirt centered with respect'to thehead at all temperatures.

HAROID H. DILLON

